Mutagenicity assays using the Salmonella typhimurium mutant strains were initiated. The metabolites of various carcinogens, e.g., N-2-fluorenylacetamide (FAA), 2,4-diaminoanisole (DAA), 1,2-dimethylhydrazine (DMH), N-nitrosomethylurea (NMU), were obtained by administering them to rats and collecting the rat urines, then testing for mutagenicity with the Ames strains. When carcinogen inhibitors were administered with the carcinogens to rats, e.g., acetanilide against FAA, the mutagenicity of the urine increased; nonantagonists did not increase the mutagenic activity. Feeding tyrosine was found to increase the mutagenicity of urine excreted by DAA-treated rats.